This application claims priority of European Patent Application No. 98303417.4, which was filed on Apr. 30, 1998.
This invention relates to channel estimation methods and apparatus in mobile radio communications, which adaptively compensate for channel distortion on a block-by-block basis.
In digital mobile radio communications, transmission channels suffer from severe distortion due to frequency selective fading. In addition, channel characteristics are normally time-varying due to the relative motion of fixed and mobile stations. Inter-symbol interference (ISI) is one of the primary impediments to reliable estimates of the transmitted data. In order to allow for reliable transmission, the receiver must be able to estimate and compensate for channel distortion on a block-by-block basis. Equalization schemes usually employed in modern mobile communications rely on an estimate of the channel, generated from a known training sequence, inserted into the transmitted signal block. Equalization may be improved by means of decision feedback, as described in K. H. Chang and C. N. Georghiades in xe2x80x9cIterative Join Sequence and Channel Estimation for Fast Time-Varying Inter-symbol Interferencexe2x80x9d, Proc. Intern. Conf. Commun. Pp.357-361, June 1995.
There is thus a need for an equalizer with improved performance and which reduces the effects of error propagation.
According to a first aspect of the invention there is provided a method of estimating channel impulse response in a signal transmitted over a channel in a communication system characterized by obtaining a priori knowledge about the transmitted signal and using the transmitted signal and the said a priori knowledge to choose an estimate of channel impulse response which minimizes the expected distance between the transmitted signal and a reconstructed signal.
In hard decision feedback, the decisions are assumed to be correct, and they are used in addition to a training sequence. Unfortunately, erroneous decisions may cause error propagations. With soft decision feedback, on the other hand, the decisions are usually in the form of log likelihood ratios (LLR).
A detailed description of the invention, using by way of illustration a practical digital radio receiver, is described below with reference to the following figures in which: